Highly linear and efficient RF power amplifiers are critical components in modern radio communications systems. Feed-forward linearization is a known linearization technique, which is especially used in MCPAs in order to fulfill extremely high linearity requirements, e.g. for third generation (3G) radio communication systems using high RF frequencies in the order of GHz. It is difficult to implement a cost-effective and time-efficient feed-forward linearization technique due to stringent requirements for accurate matching of the time-delay loops in the MCPAs. Because of common deviations of RF components fabricated in standard processes, the accurate time-delay match in the MCPAs with feed-forward linearization implementation is today made manually by, e.g., cutting or soldering the length of delay-line cables or microstriplines, which is a time-consuming and costly process. Such a manual testing process may often lead to lower linearity performance of the MCPAs due to limited manual capability of making an extremely fine-tuned match, especially in mass production. Besides, when the MCPAs in fields are deployed with radio base stations, they have to be regularly tested and possibly re-configured in order to compensate for variations in the characteristics of the MCPAs due to component aging as well as environmental effects. This is a difficult task for mobile telephone operators who may have no specially trained technicians.
In spite of introducing tunable delay lines for feed-forward power amplifiers, and thus obtaining the possibility to implement an automatic time-delay adjusting method, there are practical difficulties due to, e.g., the extremely high linearity requirements for the feed-forward MCPAs. That prevents the use of existing semiconductor tunable components.
Using delay filters to replace conventional transmission delay lines, such as coaxial cables, have also been proposed, which may provide size, integration, and possible cost advantages. There are, however, some practical problems with regard to, e.g., the bandwidth limitation of the delay filter. The intrinsic linearity characteristic and the time-delay tunability of the delay filter in a relatively wide tuning range required by the feed-forward MCPAs are examples of such problems.
The U.S. Pat. No. 6,281,838 discloses a phased array antenna system that employs Single-Pole Single Through (SPST) RF MEM switches and transmission lines to provide true time-delays in order to steer the antenna beam. However, the antenna system described in U.S. Pat. No. 6,281,838 does not provide an automatic time-delay adjustment and, in particular, not in connection with a MCPA.
The U.S. Pat. No. 5,828,699 discloses an adaptive time-delay adjuster. The time-delay elements are always connected to the transmission line path (see FIG. 1) even when the time-delay elements are switched off. That affects the local impedance and RF characteristics of the entire delay line, which results in local impedance mismatching and reflection, and certain filter functions block certain bands of signals passing through the delay line. Thus, the time-delay adjuster disclosed in U.S. Pat. No. 5,828,699 is not adapted for high RF frequencies.
An object of the present invention is to achieve automatic time-delay adjustment with a high precision, for use in high frequency applications.